1. Field of the Invention
The present invention relates to positive hole injecting materials that can increase the amount of injected positive holes; highly reliable organic electroluminescent (EL) elements that can provide higher luminance with lower voltage and exhibit lower leak currents at negative biases; and higher performance organic EL displays that utilize the organic EL elements.
2. Description of the Related Art
Organic EL elements have features such as self-luminousness and rapid response, and are expected to apply to flat panel displays. Such organic EL elements are attracting worldwide interests in terms of larger area elements that are capable of emitting light at lower voltages such as 10 V or less, since two-layer type or laminated type organic EL elements were announced that were prepared by laminating an organic thin layer or positive hole transporting layer and an organic thin layer or an electron transporting layer (see “C. W. Tang and S. A. VanSlyke, Applied Physics Letters vol. 51, 913 (1987)”, for example). The organic EL elements of the laminated type typically have a basic construction of positive electrode/positive hole transporting layer/light emitting layer/electron transporting layer/negative electrode; in addition, the function of the light emitting layer may be combined to the positive hole transporting layer or electron transporting layer as in the two-layer type.
Conventionally, positive electrodes in organic EL elements have been formed from conductive transparent substance, typically from indium tin oxide (ITO). However, organic EL elements that are manufactured by forming a light emitting layer on a positive electrode formed of ITO etc. suffer from insufficient amount of injected positive holes and non-actuating at lower voltages.
Therefore, various trials have been made to actuate organic EL elements at lower voltages through oxidizing the surface of positive electrode by UV irradiation in oxygen gas or by oxygen plasma so as to increase the amount of injected positive holes. Further, a positive hole injecting layer formed from a copper phthalocyanine layer etc. was proposed to intervene between a positive hole transporting layer and a positive electrode to increase the amount of injected positive holes and to actuate the organic EL elements at lower voltages (see Japanese Patent Application Laid-Open (JP-A) No. 2000-30867). Further, a multi-layer electrode was proposed as a positive electrode of organic EL element that comprises a carbon fluoride layer and a conductive layer on a substrate in turn (see JP-A No. 11-219790).
However, these proposals suffer from some problems still in that the effect is not sufficient yet to increase the amount of injected positive holes; leak currents flow when negative voltages are applied; diode properties are not sufficient; reliability is not sufficient; and wettability is not sufficient with the adjacent layers. Accordingly, such desirable organic EL elements have not been achieved yet that can increase the amount of injected positive holes, actuate at lower voltages, exhibit lower leak currents at applying negative voltages, perform proper diode properties, exhibit appropriate adhesive ability with the adjacent layers to prevent delamination, and provide sufficient reliability.
The object of the present invention is to solve the above-described problems in the art and to achieve the following purposes; namely, the purposes of the present invention is to provide materials for a positive hole injecting layer that can increase the amount of injected positive holes; highly reliable organic EL elements that can provide higher luminance with lower voltage and exhibit lower leak current at negative biases and appropriate diode properties; and higher performance organic EL displays that utilize the organic EL elements.